This invention is directed to a method of dewatering animal farm wastewater using very high molecular weight cationic dispersion polymers.
The agricultural community, and animal farms in particular, faces never-ending concerns regarding the disposal and spreading of agricultural waste onto land. As new environmental legislation is enforced, the agricultural community is finding itself under increased scrutiny. Among other matters, the agricultural community is concerned with the very high pollution potential of farm slurries.
In addition to the lowering of pollution, the agricultural community strives to reduce slurry volume. The reduction of slurry volume provides environmental advantages. For instance, the reduction of slurry volume can alleviate the need for large holding tanks. Moreover, since a reduced slurry volume lessens the number of times heavy machinery passes over the land, spreading is less time consuming and less arduous on the land. Still further, the reduction of slurry volume reduces the possibility of runoff onto local watercourses.
Although many methods have been proposed for the treatment of agricultural waste such as odor control of slurries, use of high pressure filtration systems for removal of suspended solids from the slurry, aerobic and anaerobic digestion etc., none of the proposed methods are being implemented on any significant scale.
Therefore, the current options for waste treatment are limited. At present, farm waste is generally recycled via lagoon collection and intermittent slurry application onto land. However, as a result of the new Environmental Protection Act (1991), the National Rivers Authority Proposals for Protection of Groundwater, and the new Control of Pollution Regulations (1991), restrictions are now being placed upon how waste is collected and reused. Conceivably, regulations will be enacted that control the use of such agricultural waste. For instance, regulations will likely place a ban on the application of waste to certain top grade agricultural lands.
If such regulations are enacted, the agricultural waste will have to be disposed of off-site resulting in substantial expense resulting from shipment of slurry to off-site disposal sites.
Therefore, a need exists for a treatment system that addresses all the environmental concerns presented by animal farm waste, while at the same time not involving high capital costs.
A method for purifying agricultural waste comprising treating the waste with a latex or dispersion polymer flocculant, an organic coagulant alone or in combination with an inorganic coagulant and one or more precipitants is disclosed in U.S. Pat. No. 5,531,907. However, there is an ongoing need to improve the efficiency and effectiveness of flocculants for dewatering animal farm waste water.
We have discovered that very high molecular weight cationic dispersion polymers that incorporate specific amounts of hydrophobic cationic monomers are far more efficient at dewatering animal farm wastewater than the cationic dispersion polymers disclosed to date.
Accordingly, in its principle aspect, this invention is directed to a method of dewatering animal farm waste water comprising:
(1) adding an effective amount of a cationic dispersion polymer having an RSV of at least 24 dl/g, the dispersion polymer prepared by polymerizing under free radical forming conditions in an aqueous solution of a water-soluble salt in the presence of a particle stabilizing agent
(i) from about 20 to about 90 mole percent nonionic monomers and
(ii) from about 10 to about 80 mole percent of cationic monomers wherein the cationic monomers are selected from at least one hydrophilic cationic monomer and at least one hydrophobic cationic monomer and wherein less than 50 percent of the cationic monomers on a molar basis are hydrophobic monomers,
(2) mixing the waste water and the cationic dispersion polymer to form a mixture of water and flocculated solids and
(3) separating the flocculated solids from the water.
The very high molecular weight cationic dispersion polymers of this invention are superior to previously disclosed cationic dispersion polymers for dewatering animal farm wastewater in a number of aspects. For instance, higher molecular weight improves polymer performance by the more effective bridging of suspended particle contaminants in the wastewater, thus forming larger flocs.
Furthermore, it is known in the art that inclusion of a hydrophobic monomer component facilitates the smooth precipitation of fine dispersion particles during a polymerization and reduction of the level of hydrophobic monomer increases the difficulty of preparing stable dispersion polymers. We demonstrate herein that it is feasible to simultaneously prepare very high molecular weight cationic dispersion polymers while reducing the hydrophobic monomer component of the polymer to less than 50 percent of the total cationic monomer content.
The combination of very high molecular weight and reduced hydrophobic monomer content in the cationic dispersion polymers of this invention results in flocs having enhanced strength and stability. Strong and large flocs are critical to high performance in dewatering applications where higher levels of mechanical system shear are experienced.